In an optical device used for a common video camera, solid-state image sensing device are arranged along the optical axis in the order of, from the object side, a combined optical system, an optical low-pass filter, a color separation filter, a CCD (Charge Coupled Device), an MOS (Metal Oxide Semiconducter), etc. Among these elements which constitute an optical device, the optical low-pass filter serves to filter optical spurious signals detected by an image sensing device, thereby preventing deterioration of the image quality of the video camera. Conventionally, if the color information of an object resembles the pixel pitch of the image sensing device like the CCD (for example, in the case of plaid), when such an object is recorded, spurious signals which are different from the actual image information may be generated in the image sensing device, which causes the colors of an output image to blur (Moire phenomenon). The optical low-pass filter is employed to cut off and attenuate the spatial frequency components associated with such spurious signals. Recently, miniaturized video cameras, etc., which contain smaller space for accommodating the optical device, have created a demand for housing the optical device effectively in a limited accommodation space. For example, in the disclosure of Japanese Patent Laid-open Publication No. S59-11085, the package for accommodating the CCD is directly sealed with an optical low-pass filter, without a cover. Incidentally, electronic still cameras are employed for image acquisition by a personal computer or for a videophone. As the CCD to be used in this application, pixels are arranged on a square lattice, in an equal pitch both horizontally and vertically.
With regard to the CCD for electronic still cameras, the optical low-pass filter employed with the CCD should be made in a square shape that matches the shape of the CCD. Besides, when the optical low-pass filter is mounted, it should be correctly oriented lengthwise and widthwise. If the length and width of the optical low pass filter is wrongly oriented, it cannot ensure desired filter characteristics nor remove spurious signals, only to provide a defective product. In order to indicate the correct lengthwise/widthwise orientation, traditional technologies include a marking step of forming the optical low-pass filter with a notch or the like in the direction of the optical axis. However, this additional step has inevitably raised the cost.
On the other hand, the optical device has been investigated for another use as a simple camera at the mobile terminal. In this case, the camera incorporates only the necessary parts among the above-mentioned optical device elements (i.e. combined optical system and solid-state image sensing device). The optical device for this application is generally equipped with a cover. The cover is fitted with a step which is formed at the opening in the package and which has a shape fittable with the cover.
From another aspect, the solid-state image sensing device particularly tends to produce unnatural colors, owing to the transmission of infrared rays. For this reason, the package for accommodating the solid-state image sensing device is made of a non-light-transmissive material, and the cover is provided with an IR-cut coating. In this structure, the step and the cover are attached by a UV-curable resin adhesive, so as to shorten the temporary curing time and thus to avoid the adverse influence by heat.
Although this optical device has the arrangement for cutting off the infrared rays, the IR-cut coating applied to the cover also shuts off ultraviolet rays. After all, the UV-curable resin adhesive used for attachment of the step and the cover cannot be cured enough, resulting in sealing failure at the cover.
In order to deal with this problem, traditional technologies have made some considerations. In one arrangement, a mask jig is placed on the cover, so that the IR-cut coating is deposited except where the UV-curable resin adhesive is applied. Alternatively, a thermosetting resin adhesive is employed instead of the UV-curable resin adhesive.
Nevertheless, the former arrangement increases the production steps, because the covers need to be processed one by one. Besides, the mask should be individually prepared for covers of various sizes. In the end, the former arrangement shows poor production efficiency, despite the inevitable cost increase. As for the latter arrangement, the thermosetting resin adhesive requires a few minutes for temporary curing, whereas the UV-curable resin adhesive cures in just a few seconds. Under longer exposure to heat, the solid-state image sensing device cannot avoid unfavorable influence by heat.
The present invention is to solve the above-mentioned various problems concerning the optical devices.
The first object is to provide an optical device which can reduce the weight and size of the optical device, which can be produced easily, unaffected by the lengthwise/widthwise orientation of mounting the optical low-pass filter, and which does not degrade the filter function of the optical low-pass filter.
The second object is to provide an optical device which can protect the solid-state image sensing device from the adverse influence caused by infrared rays, and which can be sealed precisely.